Digital pre-distortion (DPD) technique is widely used in power amplifier (PA) design to improve the performance and efficiency of the PA in electronic apparatuses such as, for example, wireless communication apparatuses. To provide a robust DPD solution over process, temperature and voltage variations, closed-loop DPD system is often employed. In a closed-loop DPD system 1600 as shown in FIG. 16, an RF loopback path (RFLB) is used to replicate signal distortion in the PA output signal, which is fed into a DPD calibration engine. The DPD calibration engine then processes this distortion to generate a pre-distorted signal. This DPD compensation is fed back to the PA to compensate for signal distortion during normal operation. To compensate the PA accurately, it is essential that the RFLB path replicates signal distortion in the PA with high fidelity.
In actual implementation of such closed-loop system, however, there are often unwanted couplings to the RFLB path, thus resulting in degradation in the fidelity of the RFLB signal. The unwanted couplings can arise from, for example, supply network, non-intended signal paths, substrate coupling, on-chip magnetic coupling, bond wire coupling, package coupling and printed circuit board (PCB) coupling. To illustrate the issue due to unwanted couplings, consider a generalized case 1700 shown in FIG. 17 where there is a forward coupling and a backward coupling between the PA input and the RFLB signal with transfer function A1ejθ1 and A2ejθ2, respectively. As shown in FIG. 17, the transfer function of the PA is GPA and the transfer function of the RFLB path is GRFLB.
During normal operation, the RFLB path is off and the PA transfer function can be expressed as Equation (1) below, where Vin represents an input signal of the PA and VPA represents an output signal of the PA:
                                          v            PA                                v            in                          =                  G          PA                                    (        1        )            
During RFLB, the RFLB path is on and RFLB signal can be expressed as Equation (2) below, where Vrflb represents an output signal of the RFLB path:
                                          v            rflb                                v            in                          =                                                            G                PA                            ⁢                              G                RFLB                                      +                                          A                1                            ⁢                              e                                  j                  ⁢                                                                          ⁢                                      θ                    1                                                                                            1            -                                          A                2                            ⁢                              e                                  j                  ⁢                                                                          ⁢                                      θ                    2                                                              ⁢                              G                PA                            ⁢                              G                RFLB                                                                        (        2        )            
Accordingly, the RFLB signal does not replicate the PA signal faithfully as a result of unwanted couplings. Undesirably, the distortion of the PA cannot be compensated accurately by the DPD calibration engine of the closed-loop DPD system.